Key Takeaways
- LCP measures loading: the largest visible element should render within 2.5 seconds
- INP (replacing FID) measures interactivity: users should interact within 200ms
- CLS measures visual stability: elements should not shift unexpectedly during load
- Image optimization is often the biggest LCP improvement opportunity
- Use field data from Chrome UX Report for measuring real user experience, not just lab scores
Largest Contentful Paint (LCP) Optimization
LCP measures when the largest visible content element finishes rendering. For most pages, this is a hero image, heading, or video poster. Optimizing LCP requires addressing the entire loading chain.
| Optimization | Impact | Implementation Effort |
|---|---|---|
| Preload LCP image | Major | Low (add link rel=preload) |
| CDN delivery | Major | Medium (configure CDN) |
| Server response time (TTFB) | Major | Medium-High |
| Image compression + WebP | Significant | Low-Medium |
| Critical CSS inlining | Moderate | Medium |
| Remove render-blocking resources | Moderate | Medium |
Identify your LCP element using Chrome DevTools Performance panel. Optimize that specific element and its loading chain rather than making generic improvements.
Interaction to Next Paint (INP) Optimization
INP measures responsiveness by tracking the latency of all user interactions throughout the page lifecycle. The worst interaction determines the INP score.
- Break long JavaScript tasks into smaller chunks using requestIdleCallback
- Defer non-critical third-party scripts (analytics, chat widgets)
- Use web workers for CPU-intensive computations off the main thread
- Implement code splitting to load JavaScript only when needed
- Debounce or throttle event handlers for frequently firing events
Third-party scripts are the most common INP offender. Audit every third-party script's impact on main thread blocking time and remove or defer any that are not essential.
Cumulative Layout Shift (CLS) Prevention
CLS measures unexpected layout shifts during the page lifecycle. Every shift where elements move without user interaction contributes to the CLS score.
- Set explicit width and height attributes on all images and videos
- Reserve space for ad slots and dynamic content with CSS aspect-ratio or min-height
- Use font-display: optional or preload fonts to prevent FOUT-induced shifts
- Avoid inserting content above existing visible content
- Animate using transform instead of properties that trigger layout
| CLS Cause | Fix | Priority |
|---|---|---|
| Images without dimensions | Add width/height attributes | Critical |
| Ads loading late | Reserve ad slot space with CSS | High |
| Web fonts loading | Preload + font-display: swap | Medium |
| Dynamic content injection | Reserve space or use transform | Medium |
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